One Biology Professor, Several Graduate Students and the Parents of a Pear Tree

Theresa Culley raised caterpillars in her playhouse as a child in Southern California. She moved on to breeding molly fish at age 10, discovering that crossing black fish with white fish resulted in Dalmatian-colored fish in Mendelian ratios.

As an assistant professor in the Department of Biological Sciences, her scientific tools and surroundings are much more sophisticated – and her work is drawing a lot of attention.

Theresa Culley, pictured in the UC greenhouse.

This summer, she talked about her latest area of focus – pear trees – as an invited participant at the Botanical Society of America meeting in Chico, Calif., in a symposium on “Hybridization as a stimulus for the evolution of invasiveness in plants.” Culley’s talk centered on work she and a graduate student, Nicole Hardiman, are conducting to determine why a common pear tree often planted in yards started cropping up, unplanted, along local roadways over the past few years.

Often called the Bradford pear – there are other types, such as the Cleveland Select – it’s one of the most popular trees in the Midwest, Culley said.

While her background is in plant reproductive biology and ecology, over the past 10 or so years Culley became interested in also incorporating genetic tools into her research. This summer, she began using a new Beta version of a software program called NewHybrids, developed by Eric Christopher Anderson and Elizabeth Thompson at the University of Washington a few years ago.

“The advantage of this program is primarily that it can look at several samples, plant or animal, and tell which samples belong to one parental group, which belong to the other parental group, and which are the offspring (children, grandchildren, etc.) of the two groups,” Culley said.

“This type of approach is ideal for looking at an organism in which you think the two species or populations are coming together and crossing in forming what we call a ‘hybrid swarm,’ but you are not sure which samples are the hybrids and which are the parents."

The program uses a statistical Bayesian clustering approach to tell the probability that a given sample belongs to either parents, or if it is an offspring, she said.

“Furthermore, if the sample is an offspring, the program tells the probability that it is a child - called an ‘F1’ – a grandchild after two siblings cross – an ‘F2’ – or a cross back to a parent, a ‘Backcross,’” Culley said.

The program is “very graphical and shows you the status of a simulation throughout each iteration,” she said. “This way you can easily tell if the software is on the right track if you add in known samples of two parents."

Ornamental pear trees are common in this Cincinnati-area subdivision.

Culley became involved with the software program as she and her students tried to identify the parents of the wild trees, she said. “Our hunch was that the parents were cultivated trees that people had planted in their yards nearby but we wanted to be sure,” she said. “Using basic genetic techniques, much as what is used in forensics, we are now able to look at the DNA of wild trees and match it up with possible parent trees in the area. It turns out that many of the wild trees are indeed the offspring of different cultivated trees in the area.”

For example, she said, a Bradford tree in someone’s yard may cross-pollinate a Cleveland Select pear tree in the neighbor’s yard. Resulting seeds, she said, are then eaten by birds and defecated along the roadway, where they germinate and grow.

“It is just a little too early to tell how well our results and analysis have been received because our study is really the first that I am aware of that explores the idea that crossing between genetically different populations within a single species can result in an invasive plant,” she said.

“Many studies have indicated that crossing between different plant and animal species can result in an invasive problem, but the single species way is an entirely new avenue of research.”

Wild pear trees do begin to become a nuisance, she said, because about 30 percent of them are thorny along the trunk and stems, and they tend to grow in clumps - resulting in groups of thorny thickets.

This young wild pear tree was found growing in a restored wetland and prairie area at Miami Whitewater Forest Park.

"They are also very difficult to remove once they get established in an area. This is a big problem already in areas of the South, but not yet here in Ohio because the wild pears are just getting started," she said.

Culley, who’s beginning her fifth year at UC, has been working on the pear project for the past three years. Last year, she received a $325,000 grant from the United States Department of Agriculture to fund the research.

Originally a pre-vet major as an undergraduate, she changed her career because of participation in an undergraduate program in which she was able to work in Hawaii with an endangered plant species, said Culley, who earned her PhD at Ohio State University.

“This is one reason why I like to have undergraduate researchers in my own lab now, in addition to graduate students,” she said.